Iodinated derivatives of the hormone avian pancreatic polypeptide

The effect of C-terminus and N-terminus iodination on avian pancreatic polypeptide (APP) binding to its chicken brain receptor

In previous studies, membranes from chicken gastrointestinal tissues failed to bind appreciable levels of 125I-APP labeled at the C-terminus. In order to address the suggestion that this was due to steric hindrance of the critical C-terminus, N-terminally labeled 125I-APP was utilized in in vitro membrane binding assays. Membranes from chicken cerebellum and spleen specifically bound N-terminally iodinated APP, while those from gastrointestinal tissues including pancreas, mucosal and muscle layers of duodenum and proventriculus did not. Cerebral cortex membranes also failed to specifically bind Bolton-Hunter labeled 125I-APP. Liver membranes, which previously were shown to bind C-terminally iodinated APP with low affinity, also did not specifically bind N-terminally labeled preparations. It is concluded that the inability of membranes from gastrointestinal tissues and brain regions other than cerebellum to bind 125I-APP is not an artifact of location of iodine placement on the molecule and that both the N- and C-termini may be important for receptor binding. It is also concluded that liver APP binding sites may be structurally distinct from those in the cerebellum, and that gastrointestinal tissues may not be direct targets for APP action.

Site specific radioiodination of recombinant hirudin

Recombinant hirudin variant rHV2-Lys47 was radioiodinated using the chloramine-T method. Depending on the reaction pH, the two tyrosine residues, Tyr3 and Tyr63, responded differently to iodination but without change in total iodination yield. Of the incorporated -125 iodine 80% was located on Tyr3 at pH 7.4, but 65% was found on Tyr63 at pH 4. These distinct iodination patterns suggest the existence of a pH-dependent multimerization and/or important conformational changes in the tertiary structure with pH. Each radiotracer was purified to high specific activity by simple low-pressure chromatography including gel filtration and reverse-phase separation, both on short cartridges. The method was validated by reverse-phase and anion-exchange HPLC with on-line radioactivity detection. The iodination sites were characterized following carboxypeptidase Y cleavage coupled with radio-HPLC.

An immunochemical study of avian pancreatic polypeptide: the nature of the principal epitope

Structural features contributing to the antigenic recognition of the small globular hormone avian pancreatic polypeptide (APP) by a polyclonal antiserum have been defined using a solution phase radioimmunoassay technique. Cross-reactivity studies with PP homologues suggest that the surface residues within the alpha-helix of the peptide may be antigenic, whereas hydrophilicity and atomic mobility predictive methods implicate the molecules beta-turn region. Immunochemical data and circular dichroism measurements on a timed trypsin digest of APP indicate that the secondary structure of the alpha-helix is vital for the molecule's immunological competence. Immunoreactivities of iodinated derivatives of APP, as well as that of peptide fragments of APP and its homologues, support the importance of teritary structure involving the interaction of the polyproline and alpha helices. The highly mobile C-terminal residues 34-36 (His-Arg-Tyr-NH2) have been found by immunological analysis to be unimportant. Arginine residue 33, which has been conserved through vertebrate evolution, is a major antigenic contributor, since a large decrease in immunoreactivity, not accompanied by a significant change in conformation, was observed upon specific removal of this residue by carboxypeptidase B. These results are consistent with a "discontinuous" epitopic model for APP in which Arg-33 and exposed residues in the alpha-helix are principal components of an epitope or epitopes mediated by the secondary and tertiary structures of the molecule.